One disadvantage of satellite positioning systems (SPS) for location determination is the relatively long time needed to perform signal acquisition. Satellite vehicle (SV) signals cannot be tracked until they have first been located by searching in a two-dimensional search “space”, whose dimensions are code-phase delay and Doppler frequency shift. Typically, if there is no prior knowledge of a signal's location within this search space, as would be the case after a receiver “cold start”, a large number of code delays (e.g., ˜2000) and Doppler frequencies (e.g., ˜15) must be searched for each SV signal that is to be acquired and tracked. Thus, for each signal, up to 30,000 locations in the search space must be examined. Typically these locations are examined sequentially, one at a time, a process that can take as long as 5 to 10 minutes. The acquisition time is further lengthened if the identities (i.e., PN-codes) of the four satellites within view of the receiving antenna are unknown.
Signal acquisition is needed at least when an SPS receiver has lost the signals, which may occur, for example, after power down, or when the signal has been blocked from the receiver for some period of time. After acquiring the signals, they may be maintained or “tracked.”
Many devices such as mobile devices possess SPS functionality as an additional feature or enhancement, rather than as a primary purpose of the device. For these devices, a need to continuously track SPS SV signals would increase the cost, decrease the battery life, or reduce the functionality of the primary device (e.g., primarily functioning as a cell phone). For example, because SPS SV signals are provided at frequencies that differ from cellular telephone signal frequencies, a single receiver cannot simultaneously monitor both frequencies. In order to do so, a mobile device would need an additional receiver unit, thereby adding to the cost of the device. Moreover, the processing capability of the system would need to be increased in order to concurrently monitor both signals, which would increase both cost and power consumption. Accordingly, many such systems rarely track SPS SV signals, but rather acquire the needed signals only upon demand.
Typically, SPS-capable systems require acquisition of SPS SV signals. Some systems only occasionally require such acquisition, while others require acquisition of the SPS SV signals each time they are needed for a SPS function. The need for signal acquisition, unfortunately, does not prevent SPS functions from being needed urgently, such as when a mobile device location is required quickly to facilitate response to an emergency. In such situations, the time delay associated with a 5 to 10 minute SPS satellite signal acquisition cold-start by a SPS/wireless terminal unit before a position determination can be obtained is highly undesirable.
In order to reduce this delay, information may be provided to aid a receiver in acquiring a particular signal. Such acquisition assistance information permits a receiver to narrow the space that must be searched in order to locate a signal. The acquisition assistance information (i.e., time assistance data), in one aspect, may include the SPS time (e.g., GPS time) or UTC time, which may provide information related to the subframe boundary associated with the SPS signals. Acquisition assistance information may also include other information about the signal, such as its PN (pseudo-noise or pseudo-random) code, frequency, modulation, and content. The provided acquisition assistance information may allow the mobile device to acquire the signal quicker. In addition to shortening the delay before a location determination can be produced, the acquisition assistance information also reduces the processing burden on the receiver, which may reduce power consumption. Systems in which receivers locate ranging signals for position location (such as GPS signals) upon demand, with the assistance of information provided from another source within the system, are generally referred to as “wireless assisted position location” systems.
In a Code-Division Multiple Access (CDMA) network, the system time for CDMA is always in-sync with GPS time. Thus, acquisition assistance information, such as GPS time may be derived directly from the CDMA system time. In a Long Term Evolution (LTE) network, system information that includes a GPS time and/or UTC time may be broadcast by an eNB by way of one or more system information blocks (SIBs), such as SIB8 and/or SIB16. However, due to an incorrect configuration by the carrier and/or due to a rogue eNB, the acquisition assistance information (e.g., the GPS time) may be invalid. A mobile device receiving and utilizing invalid time assistance data may result in a significant increase in a time to first fix (TTFF) and/or a significant increase in power consumption when the mobile device is attempting to acquire an SPS signal.